Interview Questions.

Question 1. What Is Purging?

Answer :

Purging is the system of supplying enclosures with compressed air or inert gasoline on the proper go with the flow and strain that allows you to reduce the dangerous gas within the enclosure to a safe degree. Pressurization is the method of bringing compressed air or inert fuel inside an enclosure to a strain wherein there is no ingress of unsafe gasses or combustible gas. Both purging and pressurization are required in a ClassI,gas environment. Only pressurization is needed in a ClassII, dust surroundings.

Question 2. What Is Purge And Pressurization?

Answer :

Purge and pressurization allows you to apply any electrical system inside an enclosure in a dangerous vicinity.
Before manage gadgets electricity-up electrical gadget, they first flush dangerous gas and dust from the enclosure or panel, cupboard, motor housing, and so forth., with smooth uncontaminated air. The gadget inside the enclosure does now not require approvals or certifications.
Generally, we purge with instrument-grade compressed air. The go with the flow price and length depends at the enclosure quantity and countrywide & international standards. Some agencies purge with nitrogen – you could use different inert gases but nitrogen is generally available.
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Question 3. What Is The Pressure Requirement?

Answer :

Most purging applications require a minimal enclosure stress of 0.10 inches (2.5 mm) of water. One psi is same to 27.7 inches of water. In a few instances, a minimal enclosure stress of zero.50 inches (12.7 mm) of water is required to guard in opposition to ignitable dirt. But in all cases, a better enclosure stress have to be maintained to create a reasonable protection thing. In uncommon situations, enclosure pressures as excessive as 2.5inches (63.5mm) of water may be required to offset sudden atmospheric pressure fluctuations, including those created near missile launching or off-shore drilling platforms.

Question 4. How Much Purging Gas Is Used?

Answer :

Average protecting gas intake at some stage in pressurization at a zero.10 inch (2.5mm) enclosure stress need to fall someplace among zero.1 to three.Five SCFH in line with cubic foot (2.83 to ninety nine.11 l/hr) of enclosure volume. However, use will rely upon the blanketed enclosure’s integrity and everyday strain placing. Use is also depending on the quantity and size of covers and doorways in addition to gadgets which penetrate the floor. Advanced styles of safety which include cooling or dilution may also require non-stop flow rates of 30 to 100 SCFH (849.38 to 2831.26 l/m). Purging calls for a miles higher drift fee than pressurization, but simplest for a brief period of time.

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Question five. What Kinds Of Enclosures Can Be Purged?

Answer :

Any enclosure may be purged, however enclosures offering gasketing and a couple of door fasteners are ideal. Therefore, within the absence of legitimate construction necessities for purged enclosures, groups suggest enclosures which meet or exceed the National Electrical Manufacturer’s Association score of NEMA 4 or NEMA 12.

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Question 6. What Kinds Of Devices Can Be Purged?

Answer :

Virtually any simple electrical device can be purged, of all “live” or energized components may be remoted from the surrounding environment. Devices which includes push buttons, relays, timers and programmable controllers best want to be established in a sealed enclosure. Motors handiest require a totally enclosed housing.

Question 7. What Is “purging” In The Context Of Purge And Pressurization Systems For Hazardous Environments?

Answer :

In the context of purge and pressurization structures for hazardous environments, the time period “purging” refers back to the system of presenting pressurized clean instrument air or inert gasoline into an electrical enclosure with a view to reduce or remove the risky cloth inside the enclosure to a safe (non-explosive) stage.

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Question eight. What Is “pressurization” In The Context Of Purge And Pressurization Systems For Hazardous Environments?

Answer :

In the context of purge and pressurization structures for hazardous environments, the time period “pressurization” refers back to the process of bringing the same (because the purging air) clean instrument air or inert gas underneath pressure into the electrical enclosure. The stress is sufficient to create a small high quality differential strain between the internal of the enclosure and the ambient strain outdoor the enclosure. The high-quality stress guarantees there's no ingress of risky material into the enclosure.

Question nine. What Are The Benefits And Drawbacks Of Purge Protection?

Answer :

Purge protection does not require a unique enclosure as generally a widespread industrial enclosure will work. This safety approach also addresses possible internal releases of flammable gas, vapor, or liquid. This is important for analyzers and different system that have a flammable liquid or gasoline skip through them. The best disadvantage is that it requires a deliver of defensive gas, typically air, to purge the enclosure and preserve it pressurized.

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Question 10. What Are The Benefits And Drawbacks Of Intrinsically Safe Protection?

Answer :

Intrinsic safety relies at the equipment substances being of low voltage and energy and suitable for electronic devices. The working cutting-edge of the circuitry have to be low enough not to be stricken by series resistance, which can be required to limit energy. The circuitry ought to contain low values of inductance and capacitance. Supply values exceeding 30V and 1.3W are possible but might be an increasing number of hard to certify. A specified analysis of the circuit design will possibly be essential, probable revealing a need to adjust the circuit layout in an effort to gain certification. The power and form of the enclosure isn't as vital as full manage and expertise of the circuit design.

Question eleven. What Are The Differences Between Continuous Flow Purge And Leakage Compensation Purge Methods?

Answer :

A continuous glide machine will constantly glide air through the manipulate panel. The go with the flow charge relies upon on which orifice plate you pick out. Applications are small enclosures and analyzers. A leakage reimbursement system first purges the enclosure. It then shuts itself down and simply tops off for any leakages the enclosure may additionally have. It will hold a minimal pressure within the enclosure. Continuous waft is greater reasonably priced to buy but requires continuous intake of the purge medium. Leakage compensation is extra costly up front, but these higher prices can be quick recovered thru purge medium savings.

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Question 12. What Do The X, Y, And Z Purge Methods Mean?

Answer :

Purging is used significantly nowadays for electric / device enclosures. A purge and pressurized enclosure allows smooth preservation of the internal equipment, and there is no limit to the energy that may be provided. The purge medium, normally compressed air, is used to hold the enclosure at a higher stress than the out of doors of the enclosure, preventing the entry of explosive gasoline. After remaining the enclosure, there have to be a purging to dilute any gas that may be present in the enclosure earlier than making use of the electricity, hence the term, “purge and pressurize.”

There are three varieties of purge: X, Y, and Z:

Type X Purge: Type X reduces the area type in the purged enclosure from Class I, Division 1, to non-categorized in which popular cause system can be outfitted inside the enclosure. The enclosure must be purged as a minimum four complete quantity modifications, timed automatically before the electrical interlock materials strength to the general cause equipment. On loss of stress the electricity need to be disconnected besides under special conditions.

Type Y Purge: Type Y purges reduces the location class inside the purged enclosure from a Class I, Division 1, to Class I, Division 2, where nonincendive (Division 2) device can be fitted inside the enclosure. The necessities for the operation of the Y Purge are the same as the Z Purge. The distinction is the application and contents of the purged enclosure.

Type Z Purge: Type Z purge reduces the location type within the purged enclosure from Class I, Division 2, to non-classified where trendy cause device can be outfitted inside the enclosure. The enclosure should be purged at the least four full extent changes before the electrical deliver may be applied to the general motive device. This is a guide feature finished by the plant operator. On loss of stress, there is no need to disconnect the electrical supply if the Division 2 vicinity is understood to be nonhazardous, however there ought to be a visual or audible alarm.

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Question thirteen. Which Type Of Hazardous Area Protection Is The Most Cost Effective?

Answer :

Cost effectiveness depends at the vicinity category and the provision of compressed air. Y or Z purge is maximum cost-efficient while these strategies meet the requirement. Leakage compensation will even gain fee savings inside the long term with the aid of saving at the purge medium.

Question 14. What Is The Difference Between X, Y & Z Purge Systems?

Answer :

X-rated purge systems allow uncertified system to be mounted in Zone 1 (Division 1). Usually, energy to the enclosure is switched off if pressurization is lost.
Y-rated purge structures allow Zone 2 (Division 2) licensed equipment to be mounted in Zone 1 (Division 1). Usually an alarm sounds if pressurization is misplaced.
Z-rated purge device allows uncertified device to be set up in Zone 2 (Division 2). Usually, an alarm sounds if pressurization is misplaced.
Question 15. What Kind Of “air” (defensive Gas) Is Required For A Purge/pressurization System Located In A Hazardous Environment?

Answer :

Air of normal instrument excellent, nitrogen, or different non-flammable gasoline is allowed as a protecting gas. All protective fuel is to be freed from contaminants or foreign matter and shall incorporate no extra that trace quantities of flammable vapor or fuel. (This is according to the NFPA 496 specification.)

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Question 16. If Using Compressed Air As A Protective Gas, Where Should The Compressor Intake Be Located?

Answer :

The compressor intake must be positioned in an unclassified area.

Question 17. When Does Electrical Power Need To Be Automatically Disconnected?

Answer :

X purge systems require inner power isolation on pressurization loss. Except:

If energy disconnection might also cause a greater risky state of affairs;
Safety-critical structures, e.G. Emergency shutdown, public address, trendy alarm machine; which have to continue to be powered whilst pressurization is misplaced.
Many customers require automated strength disconnection in Zone 2 & Division 2, in the event that they agree with:

Safety is improved;
Local operator response to alarms signalling pressurization loss is probable to be delayed.
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Question 18. Can The Continuous (air) Flow System Cool Electronic Equipment?

Answer :

The Continuous Flow purge device is designed to save you explosions, now not to cool system. There is not any assure that purge device will decrease temperatures in the enclosure.

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Question 19. What About Heat Dissipation?

Answer :

Internally-generated heat can deplete via the pressurized enclosure walls or through attached heat sinks. If your device does now not obtain your desired T-rating.

Question 20. Air Leaks The Most Common Problem In Pressurization Systems?

Answer :

Purge & Pressurization systems work in case your inlet materials extra air than air misplaced thru leaks. If your air supply is simply too low:

The inert fuel or air strain interior your enclosure is too low to exclude unsafe fuel or dust;
The purge procedure can not purge dangerous gas or dust from the enclosure;
You can't safely perform electrical equipment or vehicles;
The purge unit stops you or the control device firing-up anything electric.
We appreciate a few readers may be frustrated. Your plant is down. It’s viable your purge device is doing its task and stopping explosions.

For all systems test:

The inlet compressed-air strain need to be among 60 to 115 PSI, four to eight bar. The compressed-air inlet pipe should be as a minimum 3/eight″ or 10mm Internal Diameter to ensure sufficient drift. Minimize bends for a smooth air-waft.
No preventable air leaks from the enclosure or housing. E.G. Unfilled conduit seals, unused holes and so forth.
The enclosure stays well sealed. Seals have to prevent water and dust getting into the cupboard or enclosure. Normally, seals permit a little pressurized air to break out. However, if the inlet compressed air strain is/turned into too high, the strain bows out the enclosure’s door and sides. This breaks the seal so pressurized air leaks.
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Question 21. Do Barriers And Isolators Alone Make My System Safe?

Answer :

No, the apparatus to be used inside the dangerous sector should both be an intrinsically secure apparatus or a simple equipment.

Question 22. What Is A Simple Apparatus?

Answer :

Any tool which does no longer generate or keep extra than 1.5V, 100mA, and 25mW, along with inductance and capacitance.

Question 23. What Are The Factors Driving The Increased Demand For Purge And Pressurization Systems?

Answer :

The elevated demand for purge and pressurization structures is driven with the aid of various of things.

Following are a few the key factors:

The persevering with consciousness on protection in risky running locations.
The excessive-fee of designing, certifying, and installing electric equipment as either explosion evidence and/or intrinsically safe.
The expanded use of automation requiring electrical/digital controls in unsafe work environments. For example: Robotics, programmable controllers, computers, printers, calibrators, analyzers, and video displays.
The latest boom in the petro-chemical enterprise added on by using the lower fee of petroleum-based uncooked material.
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Question 24. What Is Mic?

Answer :

MIC is the Minimum Igniting Current. This is the ratio of the minimal current required from an inductive spark discharge to ignite the most easily ignitable combination of a gas or vapor, divided with the aid of the minimum present day required from an inductive spark discharge to ignite methane beneath the same take a look at situations.

In the “Class/Division/Group” machine of unsafe vicinity classifications, MIC ratios are used for Groups B, C, and D to define the varieties of unsafe gases within the surrounding surroundings.

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Question 25. What Is Mesg?

Answer :

MESG is the Maximum Experimental Safe Gap. This is the maximum clearance between  parallel metal surfaces that has been found, below special test conditions, to prevent an explosion in a check chamber from being propagated to a secondary chamber containing the equal fuel or vapor at the equal awareness. In the “Class/Division/Group machine of unsafe area classifications, MESG distance values (in mm) are used for Groups B, C, and D, to define the types of dangerous gases within the surrounding ecosystem.

Question 26. What Is Scfh?

Answer :

SCFH is the Standard Cubic Feet in step with Hour is the volume float price of a vapor at fashionable conditions of strain and temperature and humidity –14.7 PSIA, sixty eight°F, and zero% relative humidity.

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Question 27. What Is Scfm?

Answer :

SCFM is the Standard cubic feet per minute. It is the volumetric glide charge of a fuel corrected to “standardized” conditions of temperature and pressure. It is associated with the mass float price of the gas by means of a multiplicative regular which depends best at the molecular weight of the gasoline.

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Question 28. Is There A Specific Nema (countrywide Electrical Manufacturer’s Association) Enclosure Specified By Nfpa 496 For Use With A Purge/pressurization System Used In A Hazardous Environment?

Answer :

NFPA 496 does now not specify a specific NEMA rating for an electrical enclosure that uses a purge/pressurization system in a unsafe surroundings. Most manufacturers of purge/pressurization systems advocate electrical enclosures that minimally meet the rating of NEMA four or NEMA 12 enclosures respectively. If the enclosure has windows, it’s important that the home windows be shatterproof. It’s fine to make certain there's no tape over cutouts and/or openings within the enclosure.

Question 29. What Are The Advantages Of Purge & Pressurization Systems As A Method Of Protecting Electrical Equipment In Hazardous Locations?

Answer :

Following are blessings of purge & pressurization systems:

Overall safety.
Most fee effective (low value of safety/extent of enclosure.
Low preservation.
Not restrained by way of amount, extent, form, strength requirements, or physical area of electrical device.
Capable of reducing moisture, warmth, dust, and corrosion inside blanketed system.
Can enhance overall performance of device within protected enclosure.
Allows for brief and clean get right of entry to to covered system.
Adds to the lifestyles expectancy of instrumentation within electric enclosure.
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